This application relates to bearing assemblies which operate in high heat environments, and in particular, to a coating for a bearing assembly which is incorporated in a rotating anode assembly of an x-ray tube.
An x-ray tube for the production of x-ray radiation used in medical and industrial applications includes an anode and cathode within a vacuum envelope. X-ray radiation is produced when an electron beam is accelerated between the cathode and anode surface by means of high voltage. The impingement of the high energy electron beam upon the surface of a circular anode target disk produces x-ray radiation and excess heat. Typical x-ray tubes for medical applications are less than 1% efficient in converting electrical energy into x-ray radiation and can reach bulk temperatures of 1200° C.-1400° C. within the target disk.
In order to distribute the excess heat created during the generation of x-ray radiation, a rotating anode assembly has been adopted for many applications. The anode assembly is rotated about an axis such that the electron beam impinges on the outer edge, or track, of the circular target disk. This rotation ensures that each section of the rotating track is only heated by the electron beam for a short amount of time before rotating out of position to cool before rotating back to be heated again. With reference to FIG. 1A, a typical x-ray tube XTR with a rotating anode includes an anode A and cathode C. The anode A includes a target T mounted on a rotor R. A bearing and housing assembly BH is contained within the rotor R and is mounted via the bearing hub H.
The bearing and housing assembly BH of FIG. 1A is shown in greater detail in FIG. 1B. In order to provide rotation, the target disk, which is typically mounted on a rotor and driven by an induction motor, is supported by a bearing assembly 1 mounted in a bearing housing 2 which provides for smooth rotation about the axis. The rotor is mounted to a bearing hub 4. The bearing assembly typically includes at least two sets of rotating elements (e.g., balls) and is lubricated using a solid lubricant such as a soft metal to provide minimal frictional resistance. Efforts are made to protect the bearing from the intense heat of the target disk in order to prolong bearing life. However, bearing temperatures can still reach up to 450° C. in many applications. In order to withstand the high temperatures experienced in the application, the bearings and rolling elements are typically fabricated from high speed tool steels such as M62, M4, M42, T5, T15, and REX-20.
To aid in the cooling of the bearing, many applications utilize bearing housings 2 fabricated from oxygen free high conductivity (OFHC) copper and/or other copper-containing alloys. This copper bearing housing is connected through a vacuum envelope and acts as a heat sink drawing heat energy away from the bearing assembly.
In most applications, the two spaced apart sets of rolling elements are separated on the bearing shaft 3 in order to provide bearing rigidity and achieve smooth rotation. The rotator shaft may also serve as the inner raceway for the rolling elements of the bearings as shown in FIG. 1b. During operation, heat is transferred into the bearing from the bearing shaft 3 through the bearing hub 4 which is machined into, or welded to the shaft 3. The large amount of bearing heating through the shaft 3 causes the shaft to lengthen due to thermal expansion. In order to compensate for this shaft expansion, the outer rings 5 of the bearings must slide within the bore of the bearing housing 2. Failure of the outer rings to slide within the housing to compensate for the thermal expansion will cause the bearing to perform improperly (e.g., due to misalignment, incorrect preload level, etc.) resulting in premature bearing failure and a lessened service life time. Bearing failure is typically accompanied by high noise and vibration, as well as increased operating torque or reduced rotational speed. In extreme failure cases, bearing seizure and stopped anode rotation may occur, which may cause overheating of the target track and damage the anode assembly.
Due to the extreme cleanliness of the bearing components and bearing housing, along with the high temperature and high vacuum environment, it is common for bearing outer rings that are made from high speed tool steel and the bearing housing made from OFHC copper or other copper alloys to gall under the relative sliding motion and/or initiate diffusion bonding or other solid-to-solid adhesion damage. A photograph showing transfer of Cu-containing material from a bearing housing to the outer surface of an outer bearing ring after operation in an x-ray tube rotating anode assembly is shown in FIG. 2. FIG. 2 is a photograph of a prior art outer bearing ring that was installed and operated in an X-ray tube rotating anode housing. After removal from the housing, transferred material from the Cu-containing metallic housing is visible on the outer bearing ring. This illustrates the extent to which adhesive interactions between the outer bearing ring and housing occurred, which prevented relative motion between the bearing ring and the housing and therefore prevented compensation for thermal expansion in the housing. From FIG. 2, it is clear that copper-containing material from the housing adheres to the surface of the tool steel outer bearing ring so tenaciously that it is torn out when the ring is removed from the housing. Strong adhesive bonds such as these prevent relative motion between the outer bearing rings and the housing during operation such that thermal expansion cannot be compensated for and the bearing is misaligned or improperly loaded and fails prematurely. When this adhesive interaction or bonding occurs, the performance of the bearing, and as a result the X-ray tube, is severely compromised as described. A method of preventing the bonding of the bearing components to the bearing housing would be a significant improvement over the existing technology.
We are not aware of any current solutions to this problem for copper-containing metal alloy housings. The bore surfaces of stainless steel housings and steel housings have been coated with silver (Ag) to maintain relative motion between the outer bearing rings and the housing. We do not believe a silver coating solution will work for the copper housings because there is a degree of metallurgical solubility between copper and silver at high temperatures, and this may encourage diffusion or adhesive bonding between Cu, Ag, and a steel bearing outer ring. Furthermore, silver is a component in common filler metals for use in brazing copper—so silver is presumably not a preferred material for preventing bonding of Cu alloys.